Just starting you have a lot to learn, give yourself some time to get up to speed and decide what mill will meet your needs.
If you get into modding the mill a lot and have few tools you will have a long time before you have something that works. If you know you will need longer travels than a 704 offers then plan on getting the next size larger mill. After years of reading the post here I have never read a post of someone who was sorry they bought too big of a mill.
You can make accurate parts with the hobby mills but working inside a .003" window is not to be expected. Most part don't need to be that good but I don't know what astronomy parts might be.
The 704 for its size is quite nice and for smaller parts or most anything that fits on a 5" vice its good.
Your final accuracy will be determined by the overall setup and tramming, How well you ways and Gib strips fit and the slop from the ball nuts , end bearings and the cutting process and tools selection. The stock C7 screws can be good or have lots of backlash. Some couplers have lash, some end bearings have lash, some ways and gibs don't move smooth.
Their is the larger Weiss 30 for sale now and the other RF clones. If you want to do a lot of mods grab a RF-31 used. They are a great drill press and OK mills and you can make all you might need to do a custom mill. They go for $600 often and you can re sell it later for the same.